It could be an unidentified foraminiferan. However, consider the definition below:
(from
http://www.reefkeeping.com/issues/2003-07/rs/index.php)
"Although foraminifera are often thought of as simply being amoebas possessing an outside shell, there is more to them than that. Not the least of the problems with such a simplistic approach is that the shell is actually interior to the outer cell membrane that constitutes the outer surface of the organism. Nonetheless, these shells act as support and protection for the majority of the protoplasm that constitutes the foram's body. Three basic shelled foram types may be recognized, defined on the basis of differences in their shell, and recently a few naked forams have been found as well. The naked forams are unusual for a second reason, that being they are found in fresh water. All other forams are marine. Foraminifera with the first skeletal type are called agglutinated or arenaceous forams. They glue sand and other materials together to form an irregular, often star- or tree- shaped structure. These organisms are very common in some coral reefs, particularly in areas where sponges are common. In these areas they may form "spicule trees;" three-dimensional structures made of sponge spicules glued together extending up into the water column. However, these irregular, or agglutinated, forams really come into dominance in the deep seas. Here some species may get large, about the size of dinner plates, perhaps larger. Some species extend up off the bottom as a tree-like shape and have been documented to snare and eat fish or small shrimps. Others form networks of root-like growths that may cover large areas. None of this group has, to the best of my knowledge, been seen in aquaria, and in fact, only a few individuals of these deep- sea groups have ever been seen alive."
It doesn't fit because:
This one is "out of the shell" but not in fresh water.
If I am wrong and it does have a shell, it is not "three-dimensional" and so would not be like the known aborescent foraminiferans. Also the author states "None of this group has, to the best of my knowledge, been seen in aquaria, and in fact, only a few individuals of these deep- sea groups have ever been seen alive."
I've looked into the foraminifera idea before. There are a lot of fossil forms as well, but none really fit the bill. Still this type of ameoba is one of the best options.
